What is the name of the lining of body cavities and passages that communicate with the environment outside the body?

The lining of body cavities and passages that communicate with the external environment is referred to as mucous membranes. These membranes play a crucial role in protecting the underlying tissues by secreting mucus, which helps to keep surfaces moist, traps pathogens, and facilitates the movement of materials, such as food through the respiratory and gastrointestinal tracts.

Mucous membranes are found in various locations, including the respiratory passages, digestive tract, and urogenital system, where they intersect with the body's environment. This protective function is vital, as these areas are often exposed to bacteria, viruses, and other environmental factors.

In contrast, serous membranes are associated with closed body cavities and do not communicate directly with the outside environment. Epithelial membranes encompass a broader category, which includes both mucous and serous membranes among others. Connective tissues, while essential for supporting and structuring various parts of the body, do not serve as linings for cavities or passages communicating with the external environment.